ARM Cortex M1 Gowin FPGA SWD

This tutorial explains how to debug the ARM Cortex M1 MCU instantiated in a Gowin FPGA using Serial Wire Debug (SWD) with a Blackmagic Probe (BMP).

Blackmagic Probe Setup

This tutorial assumes you are using a Blackmagic Probe (BMP) with the akiel/cortex-m1 branch of the blackmagic project.

Building the Blackmagic Project

To build the Blackmagic project for loading onto the BMP, use the following commands:

meson setup build --cross-file=cross-file/native.ini -Dtargets=cortexm,stm -Ddebug_output=true --reconfigure

Loading Firmware onto the BMP

Load the built firmware onto your Blackmagic Probe using dfu-util:

sudo dfu-util -d 1d50:6018,:6017 -s 0x08002000:leave -D blackmagic_native_firmware.bin

Building a Low-Cost Blackmagic Probe with STM32F103 ‘Blue Pill’

If you don’t have an official Blackmagic Probe, you can convert a cheap STM32F103 “Blue Pill” development board into one. This allows you to use its onboard SWD pins to load the custom Blackmagic Probe firmware.

Prerequisites

You will need an existing SWD programmer (e.g., an ST-Link V2 clone, another Blackmagic Probe, or a J-Link) to flash the initial Blackmagic Probe firmware onto the Blue Pill.

Blackmagic Firmware for Blue Pill

Clone the Blackmagic Project: Ensure you have the akiel/cortex-m1 branch of the blackmagic project cloned.
Build Firmware for STM32F103: Navigate to the blackmagic project directory and build the firmware specifically for the F103RC (a common chip on Blue Pills) target. This command might vary slightly depending on your meson setup:
```
meson setup build_bluepill --cross-file=cross-file/bluepill.ini -Dtargets=cortexm,stm -Ddebug_output=true
```
Then build it:
```
meson compile -C build_bluepill
```
The resulting firmware .bin file will typically be found in build/blackmagic_bluepill_firmware.bin or similar. You might need to check the build output for the exact path.

Flashing the Blue Pill

Connect your external programmer (e.g., ST-Link) to the STM32F103 Blue Pill’s SWD pins:

SWDIO to SWDIO
SWCLK to SWCLK
GND to GND
3.3V to 3.3V

Use a tool like st-flash (for ST-Link) or openocd to flash the generated blackmagic_f103rc_bl.bin file.

Using st-flash (assuming ST-Link V2):

st-flash --serial 18006000010000543931574E write blackmagic_bluepill_firmware.bin 0x8002000

(Note: The --serial number will vary for your specific ST-Link. The flash address 0x8002000 is consistent for these STM32F103 Blue Pill devices, suggesting a bootloader is likely living at 0x8000000.)

After flashing, the Blue Pill will act as a Blackmagic Probe. You can then connect it to your target (e.g., Gowin FPGA) and follow the “Debugging with GDB-Multiarch” steps above. You might need to disconnect and reconnect the Blue Pill after flashing for it to enumerate correctly as a BMP.

Identifying the BMP Device

After loading the firmware, connect your BMP. You can identify the /dev device it pops up as by checking your kernel messages:

sudo dmesg | grep tty

Look for a ttyACM or similar device.

An example dmesg output might look like this:

[549547.203242] usb 7-1.3.4.3: USB disconnect, device number 34
[549547.433722] usb 7-1.3.4.3: new full-speed USB device number 35 using xhci_hcd
[549547.573144] usb 7-1.3.4.3: New USB device found, idVendor=1d50, idProduct=6018, bcdDevice= 2.00
[549547.573157] usb 7-1.3.4.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[549547.573164] usb 7-1.3.4.3: Product: Black Magic Probe (ST-Link/v2) v2.0.0-323-gf3b99649-dirty
[549547.573169] usb 7-1.3.4.3: Manufacturer: Black Magic Debug
[549547.573173] usb 7-1.3.4.3: SerialNumber: 8A8243AE
[549547.656267] cdc_acm 7-1.3.4.3:1.0: ttyACM1: USB ACM device
[549547.662230] cdc_acm 7-1.3.4.3:1.2: ttyACM2: USB ACM device

Debugging with GDB-Multiarch

Once the BMP is connected and identified, you can start a debugging session using gdb-multiarch.

Launch GDB-Multiarch:

gdb-multiarch <path_to_your_binary.elf>

Connect to the BMP and Interact: Inside GDB, connect to the Blackmagic Probe and perform the debugging steps. Note the use of /dev/serial/by-id/ for a persistent device path.

(gdb) target extended-remote /dev/serial/by-id/usb-Black_Magic_Debug_Black_Magic_Probe__ST-Link_v2__v2.0.0-323-gf3b99649-dirty_8A8243AE-if00
Remote debugging using /dev/serial/by-id/usb-Black_Magic_Debug_Black_Magic_Probe__ST-Link_v2__v2.0.0-323-gf3b99649-dirty_8A8243AE-if00
(gdb) mon swd
Target voltage: 0.43V
Available Targets:
No. Att Driver
 1      Generic Cortex-M1 M1
(gdb) att 1
Attaching to program: /home/akiel/GMD_workspace/softcore_fw_example/Debug/softcore_fw_example.elf, Remote target
⚠️ warning: while parsing target memory map (at line 1): Required element <memory> is missing
0x00000844 in UART_SendChar (UARTx=0x50005000, txchar=101 'e') at ../PERIPHERAL/src/GOWIN_M1_uart.c:216
216	  while(UARTx->STATE & UART_STATE_TXBF);
(gdb) load
Loading section .text, size 0x1d0c lma 0x0
Loading section .ARM.extab, size 0x30 lma 0x1d0c
Loading section .ARM.exidx, size 0xa0 lma 0x1d3c
Loading section .data, size 0x158 lma 0x1ddc
Start address 0x0000051c, load size 7988
Transfer rate: 83 KB/sec, 726 bytes/write.
(gdb) c
Continuing.

Debug Trace Output

To enable debug tracing and connect to the debug console:

Enable Debug Tracing: Use the blackmagic utility with your BMP’s serial number and verbosity level:
```
./blackmagic -s 8A8243AE -v 32
```
(Replace 8A8243AE with your BMP’s serial number)
Connect to Debug Console: Connect to the debug console using tio (or your preferred serial terminal) at 115200 baud:
```
tio -b 115200 /dev/serial/by-id/usb-Black_Magic_Debug_Black_Magic_Probe__ST-Link_v2__v2.0.0-323-gf3b99649-dirty_8A8243AE-if02
```
(Ensure you use the correct /dev/serial/by-id/ path for your BMP’s debug console interface)

Akiel Aries